A primary purpose of a Diesel Oxidation Catalyst (DOC) is to oxidise certain components of Diesel engine exhaust gas in order to meet a relevant emission standard, such as vehicular regulations including Euro 5. Particularly important reactions include oxidation of carbon monoxide to carbon dioxide, oxidation of gas phase hydrocarbons (derived from unburned fuel) to carbon monoxide and water (H2O) and—for Diesel exhaust gas—oxidation of the liquid soluble organic fraction (SOF) of Diesel particulate matter, which is derived from unburned fuel and lubricating oils.
A conventional Diesel oxidation catalyst for use in treating exhaust gas emitted from a vehicle comprises a noble metal, such as platinum or a mixture of platinum and palladium, supported on an inert high surface area refractory metal oxide, such as optionally stabilised alumina.
While platinum is particularly active amongst precious metals for promoting oxidation reactions, because it is better able to remain in its active metallic form, rather than the less active oxide form, following extended exposure to relatively high temperature lean-burn internal combustion engine exhaust gases, such as those encountered in a so-called “close coupled” position of Diesel-fuelled compression ignition engines, platinum can become oxidised. (The close coupled position is generally where an inlet to a monolith substrate carrying a catalyst is at <75 cm, such as ≦50 cm downstream of an engine exhaust manifold).
NOx absorber catalysts (NACs) are known e.g. from U.S. Pat. No. 5,473,887 and are designed to adsorb nitrogen oxides (NOx) from lean exhaust gas (lambda>1) and to desorb the NOx when the oxygen concentration in the exhaust gas is decreased. According to US '887, desorbed NOx may be reduced to N2 with a suitable reductant, e.g. gasoline fuel, promoted by a catalyst component, such as rhodium, of the NAC itself or located downstream of the NAC. In practice, control of oxygen concentration can be adjusted to a desired redox composition intermittently in response to a calculated remaining NOx adsorption capacity of the NAC, e.g. richer than normal engine running operation (but still lean of stoichiometric), stoichiometric (i.e. lambda=1 composition) or rich of stoichiometric (lambda<1). It is known that oxygen concentration can be adjusted by a number of means, e.g. throttling, injection of additional hydrocarbon fuel into an engine cylinder such as during the exhaust stroke or injecting hydrocarbon fuel directly into exhaust gas downstream of an engine manifold.
A typical NAC formulation includes a catalytic oxidation component, such as platinum, a significant quantity, i.e. substantially more than is required for use as a promoter such as a promoter in a TWC, of a NOx-storage component, such as barium, and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage from a lean exhaust gas for this formulation is:NO+½O2→NO2  (1); andBaO+NO2+½O2→Ba(NO3)2  (2),wherein in reaction (2), the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. Reaction (3) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to reaction (3) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (4)).Ba(NO3)2→BaO+2NO+3/2O2 or Ba(NO3)2→BaO+2NO2+½O2  (3); andNO+CO→½N2+CO2  (4);(Other reactions include Ba(NO3)2+8H2→BaO+2NH3+5H2O followed by NH3+NOx→N2+yH2O or 2NH3+2O2+CO→N2+3H2O+CO2 etc.).
In the reactions of (1)-(4) above, the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide and sequence of catalytic coatings in the exhaust stream.
Generally, conventional DOCs do not have sufficient activity at low temperatures for advanced future Diesel engines, such as HCCI engines. Exhaust gas from future Diesel engines are projected to have exhaust gas temperatures at least 50° C. lower than that of Diesel engines found in today's commercially available, Euro 5-compliant vehicles. Therefore, substantially improved “light-off” for HC and CO oxidation would be desirable. Conventional Diesel oxidation catalysts generally use the platinum group metal (PGM) Pt, or a combination of both Pt and Pd, each supported on high surface area metal oxide supports such as alumina, silica-alumina, zirconia, titania, or mixtures thereof.
By “light-off” herein we mean the temperature at which a catalyst catalyses a reaction at a desired conversion activity. For example, “CO T50” is a temperature at which the catalyst catalyses the conversion of carbon monoxide in a feed gas e.g. to carbon dioxide at least 50% efficiency. Similarly, “HC T80” is the temperature at which hydrocarbon, perhaps a particular hydrocarbon such as octane or propene, is converted e.g. to water (steam) and carbon dioxide at 80% efficiency or greater.
A problem with these commercially available catalyst is that a lower, less active, dispersion of PGM is obtained after exposure of the catalyst to higher exhaust gas temperatures, e.g. >300° C. Such lower PGM dispersion, caused by sintering, causes loss of active catalyst sites for the hydrocarbon and carbon monoxide oxidation reactions, and therefore typically temperatures of more than 150° C. are needed to reach complete light-off in Diesel (compression ignition) applications. The combination of Pt with Pd, possibly as an alloy, can desirably reduce Pt sintering. However, this can increase catalyst costs and can reduce Pt's renowned fresh oxidation activity (Pt is most active in its metallic state, whereas Pd is more easily oxidised).
U.S. Pat. No. 5,627,124 discloses a DOC comprising a relatively low loading (≦2 g/ft3) of platinum supported on stabilized alumina and ceria in approximately equal proportions. Alumina and ceria may be mixed together to form one layer, or may be applied as two separate washcoat layers. According to the specification, the ceria component is active for SOF oxidation. The Pt oxidises gas phase hydrocarbon and carbon monoxide. Specific examples in US '124 comprise Pt loaded on a gamma-alumina underlayer and a mixture of gamma-alumina alumina-stabilized ceria (2.5% Al2O3) in a top layer.
WO 2004/076829 discloses the use of Pt/ceria or a Pt/ceria-zirconia mixed oxide as a thermally regenerable NOx storage catalyst, i.e. no periodic changing of the air/fuel mixture fed to the internal combustion engine to rich air/fuel mixtures is necessary; reaction (4) is not actively used.
WO 01/19500 discloses regenerating a sulphur poisoned Diesel catalyst by modulating the air/fuel ratio (lambda) to 0.90 or richer for a time which is in aggregate sufficient to cause release of significant quantities of sulphur-containing species from the catalyst or catalyst components, whereby the catalyst is regenerated. The regeneration can be carried out using pulses of air/fuel ratio modulation from 250 milliseconds to 5 seconds in duration. The specific examples mention a platinum-based oxidation catalyst at 90 g/ft3 loading, but there is no disclosure of any support material on which the catalyst may be supported.
WO 2004/025093 discloses a compression ignition engine operable in a first, normal running mode and a second mode producing exhaust gas comprising an increased level of carbon monoxide (CO) relative to the first mode and means when in use to switch engine operation between the two modes, which engine comprising an exhaust system comprising a supported palladium (Pd) catalyst associated with at least one base metal promoter and an optionally supported platinum (Pt) catalyst associated with and/or downstream of the Pd catalyst wherein CO is oxidised by the supported Pd catalyst during second mode operation. The only disclosure of second running mode producing rich exhaust gas, i.e. lambda<1, is wherein the catalyst comprises a NOx absorber.
S. E. Golunski et al. published an academic paper entitled “Origins of low-temperature three-way activity in Pt/CeO2” in Applied Catalysis B: Environmental 5 (1995) 367-376.
US 2010/0221161 discloses an device for the purification of Diesel exhaust gases, which device comprises, in the flow direction of the exhaust gas, an oxidation catalyst, a Diesel particle filter with catalytically active coating, and downstream of a device for introducing a reducing agent from an external reducing agent source, and SCR catalyst. The oxidation catalyst and the catalytically active coating of the Diesel particle filter contain palladium and platinum. The ratio of the noble metals platinum and palladium in the overall system and on the individual components, oxidation catalyst and catalytically coated Diesel particle filter, are coordinated with one another in such a way as to obtain firstly an optimum NO/NO2 ratio in the exhaust gas upstream of the downstream SCR catalyst, and secondly optimum heating and HC conversion behaviour during an active particle filter regeneration.
We have now discovered, very surprisingly that by contacting an oxidation catalyst comprising platinum and a reducible oxide intermittently and momentarily with a rich exhaust gas, the oxidation catalyst can recover oxidation activity caused by the platinum becoming oxidised at higher temperatures.